The present invention relates to distance measuring apparatus and more particularly to FMCW radars and altimeters.
As is well known, there are generally two types of radar distance measuring systems, one known as a pulsed radar and the other as FMCW. In the pulsed radar technique, a series of RF pulses are transmitted towards a desired target and the receiver is operated to detect the return of the transmitted pulses as they are reflected from the target. The time delay between the transmitted and received pulses is proportional to the distance to the target. By accurately controlling the transmission and detection of such pulses, highly accurate information can be obtained with respect to the distance to the target.
In an FMCW distance measuring system, a continuous RF signal is repetitively swept by a frequency modulating signal and the resulting modulated carrier is transmitted toward a target. The FM signal is reflected by the target and returned toward the transmitter for reception. A portion of the transmitted signal is mixed with the reflected signal to produce a beat frequency indicative of the distance from the transmitter to the target. By controllng the modulation of the transmitted signal and by detecting the beat frequency, accurate readings of the distance to a target can be determined. Prior art FMCW systems are described in U.S. Pat. Nos. 4,107,679 and 4,276,549 and in the article entitled "Area Target Response of Triangularly Frequency-Modulated Continuous-Wave Radars", I.E.E.E. Transactions on Aerospace and Electronics Systems, Vol. AES-14, No. 2, March 1978.
While each of the above techniques can give an indication of distance to target, each technique has its own limitations. For example, the pulsed radar technique enables easy detection and tracking of the nearest return, thereby providing easy detection of altitude or distance changes when used as an altimeter. The pulsed technique, however, requires high transmitter power and large receiver bandwidth and is normally confined to a specific frequency which enables easy jamming in a hostile environment. In addition, the high power solid-state RF devices which are needed to generate the high power RF pulses are not as readily available as low power devices. Further, at lower altitudes, it is difficult to generate and detect the pulses necessary to make accurate readings. In automatic landing systems, this becomes a problem since accuracy at low altitudes is critical for proper control.
In contrast, the FMCW technique enables distance measuring with a lower power transmitter and narrow bandwidth receiver. Because the signal is continuously modulated or swept in frequency, it is less likely to be affected by jamming techniques. In addition, the cost and complexity is substantially reduced by the ready availability of low power solid-state RF devices. By relying on the beat frequency produced by the mixing of the transmitted and received signal, however, the accuracy of the measurement is reduced. Specifically, the beat frequency produced by FMCW returns is not a single frequency but a spectrum of beat frequencies indicative of many reflections received from a target area and representing different altitudes from that target area. Conventional detection circuits produce readings which are the average of the spectrum of beat frequencies or the centroid of the spectral return of those beat frequencies rather than the nearest return. While this averaging is helpful in automatic landing systems, it does not provide high resolution for ground or terrain avoidance systems where pop-up targets are likely to occur.
In the prior art, a variety of systems have been developed to improve the accuracy of the distance measuring provided by FMCW apparatus. These techniques attempted to improve frequency discrimination and modulation non-linearities in order to more accurately control the beat frequencies. Other techniques attempt to sweep over a range of beat frequencies and selectively filter the frequencies in order to more accurately determine individual frequencies indicating nearest returns. Such attempts, however, have met with limited success since the filter characteristics and sweep time for altitude determination are prohibitive over the desired range of altitudes. As a result, the FMCW technique has been used primarily for automatic landing systems and low altitude measurements, while the pulsed technique has been used at high altitudes or where accurate target distance is required. The only other effective compromise has been to include both a pulsed radar and FMCW radar to obtain the benefits of each during anticipated use.
As can be seen, if the accuracy of an FMCW radar can be improved to detect more nearly the frequency of nearest returns, the usefulness of an FMCW radar in more environments would be greatly enhanced. One such system for improving accuracy is disclosed in co-pending U.S. application No. 330,735 entitled "Radar Altimeter Nearest Return Tracking" by Thomas A. Ubriaco, filed Dec. 14, 1981 and assigned to the same assignee as this assignee. The technique and apparatus disclosed herein is an alternative to that referenced in the aforementioned application. The present invention has therefore been developed to improve the accuracy of an FMCW distance measuring system.